Field of the Invention
The invention relates to a high output impedance current source including a transistor that determines the output current and a control circuit for the current-determining transistor.
Developments in CMOS (Complementary Metal Oxide Semiconductor) process technology mean that the basic circuits in analog circuit engineering demand new solutions that meet strict requirements of linearity and bandwidth, for instance, despite changes in transistor properties and falling operating voltages.
Current sources can be used, for instance, as basic modules for current mirror circuits. Current mirroring generally involves a circuit arrangement in which a copy of the input current is provided at the output. The purpose of a current mirror is thus to duplicate and/or respectively amplify or attenuate an input current and to output it at the output node(s). The simplest current mirror consists of two identical transistors.
Such simple current mirrors have the drawback of a relatively low output impedance, however, and are therefore unsuitable for many applications. The current through a MOS (Metal Oxide Semiconductor) transistor is determined both by the potential at the gate terminal and by the voltage drop across the source-drain channel.
In order to obtain a high output impedance, cascoded current sources are therefore used for example. In such a current source, the source-drain channels of at least two MOS transistors are connected in series, and each of the gate terminals are set at a fixed potential. By this measure, the drain-source voltage across the current-determining transistor is decoupled from voltage changes at the output node, and the output impedance is thus increased. The disadvantage of such cascoded current mirrors lies in the fact that there must be a relatively high minimum voltage between the output node and the operating voltage terminal in order to continue to achieve the specified output impedance. If the voltage falls below this, then the MOS transistors are no longer operating in saturation mode and the output impedance of the current source drops drastically. This minimum voltage drop essentially determines how fit the current source is for use with low operating voltages.
The regulated cascode current source on which the present invention is based is an improved version of this cascoded current source. Such a current source, whose design and operation is explained in more detail in connection with FIG. 1, enables the drain-source voltage across the current-determining transistor to remain constant. By this measure, the voltage drop across this current source can be reduced to a relatively small value, where the lower limit of this voltage drop is given by the threshold voltage of one of the transistors in the control loop. The high gain in the control loop means that the current source has a high output impedance.
The circuit works on the principle that the minimum voltage drop across the current source cannot fall below the threshold voltage mentioned above. This can be problematic with extremely low operating voltages.
It is accordingly an object of the invention to provide a current source which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide a current source of the type mentioned in the introduction that has a high output impedance and that can be used at very low operating voltages.
With the foregoing and other objects in view there is provided, in accordance with the invention, a current source having a high output impedance that includes: a current-determining transistor that determines an output current; a source-follower circuit providing an output signal; a control circuit having an input receiving the output signal of the source-follower circuit; and a control transistor connected in series with the current-determining transistor. A potential is defined between the current-defining transistor and the control transistor. The current-determining transistor has a source-drain voltage. The control circuit ensures that the source-drain voltage of the current-determining transistor has and maintains a required value by applying a suitable drive to the control transistor. The source-follower circuit has an input that receives the potential between the current-defining transistor and the control transistor.
In accordance with an added feature of the invention, the source-follower circuit includes at least one transistor.
In accordance with an additional feature of the invention, the source-follower circuit includes a plurality of transistors.
In accordance with another feature of the invention, at least one of the plurality of the transistors is embodied as a current source.
In accordance with a further feature of the invention, the control circuit includes an amplifier circuit having at least one transistor.
In accordance with a further added feature of the invention, the control circuit includes at least one control transistor.
In accordance with a further additional feature of the invention, a correcting element is provided for the current-determining transistor.
In accordance with yet an added feature of the invention, the correcting element includes at least one transistor.
In accordance with yet an additional feature of the invention, the correcting element includes two transistors.
The object of the invention is obtained by a current source having a high output impedance that includes a transistor for determining the output current and a control circuit for the current-determining transistor. In order to set a low, freely selectable minimum voltage drop across the current source, the current source is provided with a source-follower circuit that affects the voltage across the source-drain channel of the current-determining transistor.
This creates a current source that is distinguished by a high output impedance and that is particularly suitable for use with low operating voltages.
The invention is based on the fundamental idea that a high output impedance can be achieved by a current-source circuit designed in the form of a regulated cascode current source, as has already been described above. The regulated cascode current source first includes at least one transistor that determines the output current, whose gate potential Vin can be used to set the useful current. A suitable control circuit is used to maintain a constant voltage Vds across the drain-source channel of this transistor, and hence ensures that the current flowing through the current-determining transistor is not dependent on the potential at the output node of the current source. Such a control circuit is explained in more detail later in the description.
A source-follower circuit has also been provided in order to solve the problem, described with reference to the state of the art for the regulated cascode current source, which is that the minimum voltage drop cannot be reduced below a certain limit.
Source-follower circuits are known in the art, and a basic circuit involves at least one field effect transistor. Advantageous embodiments of source-follower circuits are explained in more detail in the subsequent description, but without restricting the invention to the cited examples.
By using such a source-follower circuit in the regulated cascode current source, the voltage drop across the current-determining transistor can be set to any value, and thus the minimum voltage drop across the current source can be reduced. In this way, very wideband current sources with high output impedance can be realized even in low-voltage applications.
Preferably the source-follower circuit can contain at least one, but preferably two or more transistors. In its simplest design the circuit can have only a single transistor. It is also conceivable, however, that the circuit can have a far more complex design according to the requirement and application. In this case the number of transistors may be correspondingly greater. The invention is not limited to a specific number of transistors. In one advantageous embodiment the source-follower circuit contains two transistors.
At least one of the transistors can advantageously be designed to act as a current source.
In a further embodiment the control circuit can contain an amplifier circuit. This amplifier circuit preferably has one or more transistors, where the number of transistors can vary according to the properties required of the current source.
In addition, the control circuit can contain at least one control transistor.
A correcting element can advantageously be provided for the current-carrying transistor.
This correcting element can, for example, contain one or more transistors. In an advantageous embodiment, the correcting element can contain two transistors.
In the inventively designed current source, the case can arise that the current-determining transistor is no longer operating in saturation mode if the voltage drop across it is reduced too far. This has the effect that the output current may fluctuate when the potential at the output node fluctuates, and thus the output impedance is reduced. This problem can be countered using the correcting element, which increases the output impedance significantly. An example of this is explained in the following description of the figures.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a current source that has a high output impedance and that can be used with low operating voltages, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.